DNI – Direct Normal Irradiance

Did you know that Direct Normal Irradiance (DNI) is one of the most important factors for successful solar power generation? What is DNI, and why is it so important for solar power?

DNI (Direct Normal Irradiance) at a location is the amount of solar energy falling per sqm per day at that location.
The DNI for a sunny region is approximately 6 kWh/m2/day. This is the amount of sunlight energy falling on a square meter every day in a good location.

The higher the DNI, the higher the electricity produced by a solar cell.

DNI is usually measured by a pyrheliometer mounted on a solar tracker.

Questions from the curious cat

Which regions of the earth have the highest solar DNI?

Northern portions of Africa, large portions of USA and Australia, Central America and South America, Middle East, southern parts of China and India get good solar DNI. Refer to this image for more (source for the image: Greenbang)

A 2009 study by German Aerospace Center estimated how much open land in each region receives incoming solar energy of more than 2,000 kWh/m²/y (about 5.5 kWh/m2/day). The following were their conclusions:

Africa – about 13 million sq km

Australia – about 6 million sq km

Middle East – 2.6 million sq km

Central South America – 1.2 million sq km

China – 1.2 million sq km

USA – about 1 million sq km

Other parts Asia – 0.7 million sq km

Mexico – 0.37 million sq km

Central Asia – 0.16 million sq km

India – 0.11 million sq km

EU 27+ – 0.02 million sq km.

What does the solar DNI depend on?

The solar radiation that reaches on different locations of earth depends on several factors such as geographic location, time, season, local landscape, local weather etc. The Earth rotates around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth’s surface receives a little more solar energy. This can cause increases in the DNI.

When sunlight passes through the atmosphere, it is subjected to absorption, scattering and reflection by air molecules, water vapor, clouds, dust, pollutants, forest fires etc. These can also affect the DNI in a particular region.

Is the DNI constant during the day?

As you would have expected, the answer is no. The amount of sunlight varies during the day, and so does the DNI.

At what times of the day is the DNI highest?

Naturally, the DNI is highest during the times the amount of sunlight is the highest. This happens around 1 PM – 3 PM in most regions.

At what times of the year is the DNI highest?

The months during which the DNI is highest depends on the region.

Is the amount of power generated by solar cells always directly proportional to the DNI?

To a significant extent, yes. All things about technology and non-environmental aspects being equal, the extent of power generated by solar cells is directly proportional to the DNI.

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